angelfish dying

I have recently introduced 5 small angels into an established
planted tank containing corys, a pleco, and a few tetras.

After about 1 week the first one went off his food and died
within 2 or 3 days. One by one this has happened until now
the last fish is showing signs.

Fish appeared healthy , vigous and growing until stop eating and
hiding also that stingy,clearish white feces

What's going on?

Doug N